The goal of this application is to determine how T lymphocytes increase the capacity of ovariectomy (ovx) and parathyroid hormone (PTH) to expand hemopoietic stem cells (HSCs). Ovx is known to increase the number of hemopoietic cells in the bone marrow (BM) and our preliminary data show that ovx increase the number of HSCs in T lymphocyte replete but not T lymphocyte deficient mice. Preliminary data show that T lymphocytes contribute to the HSC expansion induced by ovx by expressing the costimulatory molecule CD40L and by secreting soluble IL-6 receptor (sIL-6R), a receptor subunit required for HSCs to respond to the hemopoietic cytokine IL-6. Data also show that blockade of the T lymphocytes costimulatory molecule CD40L prevents T lymphocytes from upregulating stromal cells (SCs) Jagged1 and IL-6 expression. Jagged1 is a Notch ligand which activates Notch signaling in HSCs. IL-6 is a hemopoietic cytokine. Based on these data we hypothesize that ovx expands HSCs through T cell CD40L mediated stimulation of SC Jagged1 and IL-6 production. PTH has been shown to expand HSCs by upregulating the SC expression of Jagged1 and IL-6. Our preliminary data show that PTH fails to increase the number of HSCs in T cell deficient mice and in mice lacking the T cell costimulatory molecule CD40L. Additional data show that PTH does not increase the number of HSCs in mice expressing a silent PTH receptor in T cells, implying a role for direct targeting of T cells by PTH. We also hypothesize that T cell contribute to the HSC expansion induced by PTH by inducing the activation of Wnt signaling in SCs. In Specific Aim 1 we will determine the mechanism by which T cells contribute to expansion of HSCs induced by PTH. Specifically we will investigate the role of T cell produced factors including Wnt ligands, sIL-6R, and CD40L in activating Wnt signaling in SCs and Notch signaling in HSCs. In Specific Aim 2 we will determine whether direct targeting of T cells by PTH is required for the hormone to induce the expansion of HSCs. This will be accomplished by utilizing a mouse generated in our laboratory which expresses a silent PTH receptor in T cells. In Specific Aim 3 we will investigate the role the role of T lymphocytes produced Wnt ligands, sIL-6R, and CD40L in activating Wnt signaling in SCs and Notch signaling in HSCs in ovx mice. Lack of information about the mechanisms by which ovx and PTH expand HSCs through T cells is a critical barrier to progress of the understanding the cross-talk between hemopoietic cells and bone cells. Our proposal has the potential to decrease this barrier. If the aims of the projects are achieved our understanding of the mechanism of action of two key calciotrophic hormones, estrogen and parathyroid hormone, will be increased. New data about the role of T lymphocytes in the mechanism by which estrogen and PTH regulate HSCs will increase the scope and the depth of osteoimmunology, a novel field of multidisciplinary research.